Metastable Fe${}_{100-x}$Al${}_x$ alloys have been formed by ball milling of elemental Fe and Al powders: supersaturated body-centered-cubic solid solutions for $x<~70$, and an amorphous phase for $x>\mathrm{}70\mathrm{}$. Quantitative x-ray-diffraction measurements show that the total root-mean-square displacement (rms) and the static rms in the Fe${}_{100-x}$Al${}_x$ solid solutions increase significantly with increasing Al content. The total rms at the instability point, however, reaches only 6.8% of the nearest-neighbor distance and is far below the critical value predicted by the Lindemann melting criterion, suggesting that the Lindemann melting criterion is not applicable for the solid-state amorphization. Instead, the Debye temperature of the supersaturated Fe${}_{100-x}$Al${}_x$ alloys was observed to drop by $\sim 22\%$ at the point of amorphization, implying a corresponding softening in the average shear modulus of $\sim 40\%$, which agrees with the microhardness measurements. These results strongly support the empirical elastic instability criterion for the solid-state amorphization process.

• Received 24 March 1997

DOI:https://doi.org/10.1103/PhysRevB.56.2302